Routing chromium(III) from the coating station

Purpose

To make sure that everything a coating session sends to a container is described by what is on that container — including the chromium, which is the constituent nobody writing the label is thinking about.

When to use it

Any session in which chrome alum has been used: as the finals of an emulsion, as the 2 per cent subbing dip for glass, or as a hardening bath. It runs alongside routing silver-bearing waste rather than instead of it, because the same rinse usually carries both.

It does not apply to a session hardened with potassium alum, or to the course’s default, which is no hardener at all.

Before you start

You should already have read the chromium policy, which is the one place the distinction lives: chrome alum is chromium(III), a different substance from the chromium(VI) of a dichromate, with a workplace exposure limit fifty times more permissive and neither a carcinogenicity nor a sensitisation statement notified. This procedure does not restate that argument and no page should.

On the bench: the silver-bearing waste container, standing at the wet end before the emulsion is molten; the labelled 5 per cent chrome alum stock and the syringe that belongs to it; a marker; and the waste log.

!The decision that removes this procedure entirely

The default in Part V is no hardener in the emulsion, and the route that puts no chromium anywhere is to take it. Chrome alum goes in where a lesson genuinely requires an in-emulsion hardener, or where a historical formula’s published finals already contain it. Where processing needs the layer to hold, the alternative is bath hardening with potassium alum, which carries no hazard classification at all. Decide this before the make, not while holding a jug.

Procedure

1. Before the session, say out loud whether chromium is in it, and write the answer at the top of the session’s notebook page. Every later step depends on that one sentence being available at the sink.
2. Stand the collecting container at the wet end, within reach of where you will be rinsing, before anything is warmed.
3. Keep one syringe for the chrome alum stock and let it measure nothing else, ever. Rinse it separately and store it with the stock bottle.
4. Send every rinse off the rod, the bed’s wrap, the jar, the tools and the gloved hand to the silver-bearing container. It carries silver as well as chromium, and the silver is what decides which container it goes to.
5. Do not tip a subbing dip after one plate. It is reused until it is retired, and only then is it collected — 300 mL at 2 per cent carries about 6 g of chrome alum, far more than any in-emulsion dose, so it is collected as its own recorded volume rather than poured in among the rinses.
6. Remember what a hardened layer does downstream. Because the hardener is in the coating, the developer, the fixer and the wash water from those sheets carry chromium too. Every container that receives them is in scope for step 7.
7. Write the chromium on the label in words — contains chromium(III), as chrome alum — with the strength and the date, on every container that received any of it. The waste code does not change, because the code is chosen by the waste’s source rather than by its composition; what changes is what the person who eventually receives the container is able to know.
8. Leave nothing to dry. A dried residue is a dust, which turns a liquid hazard into an inhalation one and makes the container harder for anyone to accept.
9. Cap, wipe the outside, stand the container back in its tray, and top up the running-volume line.
10. Route it by the route you checked and dated for your own authority. The chemistry above is general; what may lawfully be done with the container is local, changes, and differs between neighbouring authorities, so the disposal caveat is where this course stops and this procedure issues no instruction.
11. Write the log line.

How you know it worked

• The notebook page says, in one sentence at the top, whether chromium was in the session.
• No container that received a chromium-bearing liquid is without the word on its label.
• The chrome alum syringe has measured nothing but chrome alum stock.
• The spent dip has its own recorded volume rather than being lost among the rinses.
• Nothing from the session was left in a dish, a tray or a wipe to dry.
• The log says which project it came from and whether the hardener was in the emulsion or in a dip, because those produce different amounts in different places.

If a step fails

Step 1, you cannot remember whether the hardener went in. Assume it did, and label accordingly. Over-describing a container costs a word; under-describing one hands somebody a liquid they cannot classify.

Step 3, the syringe has measured emulsion. It is now a silver-bearing implement as well as a chromium one. Rinse it into the silver-bearing container, retire it from the stock bottle, and take a new one for the chrome alum. A shared syringe is the coating bench’s version of a shared graduate.

Step 4, you are unsure whether a rinse carries chromium. If the hardener was in the layer or in the pot, it does. This container only ever gains, and the cost of being wrong in that direction is a longer sentence on a label.

Step 5, the dip has already gone down the drain. Record the volume and the strength as your best estimate, and move the collecting container before the next plate. The estimate is what makes the entry useful later; the container’s position is what stops it recurring.

Step 7, the label has no room left. Then the label is too small for the container. Write a fresh one, carry every field across, and tape it over the old rather than adding a note to the side.

A hardened plate refuses to fix or clear evenly. That is a coating problem rather than a waste one, and it belongs to coating glass dry plates, which sets out why excessive hardening interferes with developer and fixer penetration. Do not respond by raising the dose.

What to record

The waste-log line: date opened, stream, container, date closed, running volume, route and the date you checked it. Then three columns this stream needs that the general one does not — whether the hardener was in the emulsion or in a dip, the dose as a percentage of the gelatin weight, and the number of sheets or plates processed from it. Those three are what would let somebody eventually answer the question this course cannot: how much chromium a domestic coating practice actually puts into a bottle in a year.